^8 Royal Society. 



The conclusion ns regards specific heats of electricity in the dif- 

 ferent metals, from the equation expressing thermo-electric force 

 given above, is that the specific heat of vitreous electricity is greater 

 in each metal passing another from left to right in the series as the 

 temperature rises than in the metal it passes : thus in particular, — 



The specific heat of vitreous electricity is greater in copper than in 

 platinum or in iron ; greater in brass than in platinum or in lead ; and 

 greater in lead than in platinum. 



It is probable enough from the results regarding iron and copper 

 mentioned above, that the specific heat of vitreous electricity is 

 positive in brass ; very small positive, or else negative, in platinum, 

 perhaps of about the same value as in iron. It will not be difficult to 

 test these speculations either by direct experiment on the convective 

 effects of electric currents in the different metals, or by comparative 

 measurements of thermo-electric forces for various temperatures in 

 circuits of the metals, and I trust to be able to do so before long. 



§ III. On Thermo-electricity in crystalline metals, and in metals in a 

 state of mechanical strain. 



Having recently been occupied with an extension of the mechani- 

 cal theory to the phaenomena of thermo-electricity in crystalline 

 metals, I have been led to experimental investigation on this branch 

 of the subject. The difficulty of obtaining actual metallic crystals 

 of considerable dimensions made it desirable to imitate crystalline 

 structure in various ways. The analogies of the crystalline optical 

 properties which have been observed in transparent solids, in a state 

 of strain, and of the crystalline structure as regards magnetic induc- 

 tion which Dr. Tyndall's remarkable experiments show to be pro- 

 duced not only in bismuth but in wax, thick paste of flour, and " the 

 pith of fresh rolls," by pressure, made it almost certain that press- 

 ure or tension on a mass of metal would give it the thermo-electric 

 properties of a crystal. The only case which I have as yet had time 

 to try, verifies this anticipation. I have found that copper wire 

 stretched by a weight bears to similar copper wire unstretched, ex- 

 actly the thermo-electric relation which Svanberg discovered in a 

 bar cut equatorially from a crystal of bismuth or antimony compared 

 ■with a bar cut axially from a crystal of the same metal. Thus I 

 found that : — 



If part of a circuit of copper w ire be stretched by a considerable 

 force and the remainder left in its natural condition, or stretched by 

 a less force, and if either extremity of the stretched part be heated, 

 a current sets from the stretched to the unstretched part through the 

 hot junction : and if the wire be stretched and unstretched on the 

 two sides of the heated part alternately, the current is reversed (as 

 far as I have been able yet to test, instantaneously) with each change 

 of the tension. 



I intend to make similar experiments on other metallic wires ; also 

 to try the effect of transverse as well as of longitudinal tension on 

 slips of sheet metal with their ends at different temperatures^ when 

 placed longitudinally in an electric circuit ; and the effects of oblique 

 tension on slips of metal similarly placed in a circuit, but kept with 



